Cell differentiation in the terminal tubule of fetal rat submandibular gland in organ culture

Morphogenesis and Mucus Production of Epithelial Tissues of Three Major Salivary Glands of Embryonic Mouse in 3D Culture

Embryonic mouse submandibular epithelia initiate branching morphogenesis within two days when embedded in Matrigel and stimulated by members of the epidermal growth factor family. However, it is unknown whether the end buds further branch over longer culture periods, and whether saliva-producing cells differentiate there. In the present study, we cultivated three major (submandibular, sublingual and parotid) salivary epithelia from 13-day embryos for 14 days in mesenchyme-free cultures. All epithelia continued to grow and branch to form numerous acinus-like structures in medium supplemented with neuregulin 1, fibroblast growth factor 1, and insulintransferrin-sodium selenite. Alcian blue staining to detect mucous cells showed that each epithelium differentiated via three distinct modes, as seen in normal development, although the staining intensities were weaker than in normal development. RT-PCR analysis of the amylase gene showed that no epithelia expressed amylase after 14 days of culture, which is inconsistent with the fact that only parotid epithelium does so at postnatal day 7 during normal development. These results suggest that cytodifferentiation progresses to a lesser extent in mesenchyme-free cultures than in vivo.

Effects of dexamethasone, epidermal growth factor, and retinoic acid on rat submandibular acinar-intercalated duct complexes in primary culture

Mature acini with attached segments of intercalated ducts were dissociated from the submandibular glands of rats and grown in primary culture on gels of reconstituted rat tail collagen. Screening evaluations indicated that the following new conditions promoted a substantial improvement in the survival of the cells as compared with our previously reported results: a) adding dexamethasone, epidermal growth factor, and retinoic acid to the medium, b) decreasing the fetal bovine serum in the medium to 1%; and c) adjusting the oxygen in the gas phase to 50%. A detailed evaluation, including light and electron microscopy and biochemical analysis, then provided the following observations. The acinar-ductal complexes enlarged throughout the 22-d culture period, and after 4 d sheets comprised of a one- to two-cell thick layer of acinar cells spread among the complexes. Synthesis of mucin, and its secretion in response to norepinephrine or cAMP, dropped precipitously to very low levels after 2 d. However, synthesis of DNA, general proteins, and glycoproteins dropped only transiently after 2 d, rising to levels approaching those of freshly dissociated complexes by 22 d. These data indicate that a shift occurred from the synthesis of large quantities of secretory proteins and glycoproteins, especially mucins, during the first 2 d in culture, to other materials thereafter. Overall, the new culture conditions resulted in substantial growth and survival of acinar cells through 22 d in primary culture, but the important acinar characteristic of the synthesis and secretion of mucins was essentially lost after 4 d.

Short-term primary culture of acinar-intercalated duct complexes from rat submandibular glands

Acinar-intercalated duct complexes dissociated from rat submandibular glands have been shown to be an excellent model for studying secretory responses of salivary gland components. However, they are functionally normal for only a few hours. We undertook a systematic manipulation of primary culture conditions in an attempt to extend the useful life of the complexes. The major modifications that were tested were increased oxygenation in increments to 95%; substitution of norepinephrine or carbamylcholine or both for isoproterenol in the medium; different sources of collagen for and addition of laminin, fibronectin and/or type IV collagen to the matrix gel; and varying the thickness of the collagen gel, richness of the cell suspension inoculate, and sources and concentrations of sera in the medium. Progress was monitored by light microscopic evaluation of routine sections of specimens until improved maintenance of acinar and other cells warranted carrying parallel cultures for biochemical, histochemical, and ultrastructural analyses. Best results were obtained with 90% O2, laminin in rat tail collagen gel, 10% fetal bovine serum, and 3 microM isoproterenol. Morphologically, there was good survival of acini and intercalated ducts after 1 d, with decreased acinar size being correlated with secretory response to the isoproterenol. Reorganization and considerable mitotic activity were seen at 2, 3, and 4 d, with most clusters of cells becoming much larger than the original complexes. During this period acinar cells steadily became less differentiated and their numbers decreased in proportion to intercalated duct or undifferentiated cells. However, there was good overall survival through 7 d. Biochemical analysis indicated that the cells were able to maintain significant biosynthetic activity for 4 d, with DNA, RNA, protein, and glycoprotein synthetic rates increasing over the culture period, but the secretory capacity of the cells diminished during the primary culture period, with mucin biosynthesis and secretion decreasing significantly after 1 d in culture.

The dependent and independent relationships between cytodifferentiation and morphogenesis in developing salivary gland secretory cells

The mesenchymal capsule was removed from the epithelial anlage of 15 days (unbranched) and 16 day (initially branched) fetal rat submandibular gland (SMG) rudiments. The undifferentiated epithelial portion of the SMG rudiments was placed in tissue culture and examined by light and electron microscopy and histochemistry for secretory peroxidase. The 15 day fetal SMG epithelial rudiments failed to attach, and spread over the culture dish and degenerated by 3 days in culture. The 16 day epithelial rudiments attached to the dish and the cells spread radially from the explant. Mitotic activity was minimal. Cells spreading from the 16 day rudiments underwent cytodifferentiation, giving rise to two secretory cell types: 1) peroxidase containing "proacinar cells," and 2) secretory "terminal tubule" cells. The results suggest that in the developing SMG, morphogenesis and cytodifferentiation are partially coupled but independently regulated processes. The earliest phases of morphogenesis (rudiment down growth and primary branching) seem to be required to initiate cytodifferentiation. Once initiated, cytodifferentiation can proceed in the continued morphogenesis (tissue organization) or significant amounts of connective tissue elements.